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Why do many bottom-living deep-sea fishes have small eyes and downturned mouths?

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A porcupine fish in the Andaman Sea — fish *Diodon hystrix* is a species of porcupine fish. These fishes are short and broad-bodied, with large eyes and spines covering their skin. Like puffer fishes, they inflate their bodies when threatened.

Bottom-living (benthic) fishes inhabit the ocean floor, an environment characterized by extreme pressure, cold, and a total lack of sunlight from the surface. These specialized traits are evolutionary adaptations to their specific habitat.12

The presence of smaller eyes in these species is a result of the dark, lightless conditions of the deep-sea floor, where vision is of limited utility compared to mid-water species that often possess enlarged eyes to detect bioluminescence or faint light from above.12

The downturned mouths of many bottom-dwelling fishes are adapted for their specific feeding habits. Because these fishes typically forage for food on or near the seafloor—such as invertebrates, mollusks, and crustaceans—a mouth oriented downward facilitates the efficient collection of prey from the substrate. Unlike mid-water predators that may require large, gaping jaws to capture active, free-swimming prey, benthic forms are specialized for grazing or scavenging along the bottom.13